Figure[Black to move]

Survey what you attack (as Black) and see, among other things, the possibility of QxN—but White’s queen on d1 guards the target. So turn your attention to the queen. You have no safe way to attack it, and it protects no other piece you can take; but there remains the possibility of interrupting its line of defense to the knight on h5. This you can do with Re2; the question is what follows from it. The important point of the move is that the rook is guarded by your queen from e5, so if White replies 2. QxR he loses his queen for a rook after 2. …QxQ.

But now what if White doesn’t reply QxR? Then he is faced with QxN. His best move thus is to launch an attack of his own with the threatened piece: 2. NxB. He is hoping you will recapture his knight, taking him off the hook, but you can do better. Look at the board as it then would appear; look for any checks you then would have; find Re1+, a rook fork of White’s king and queen. White is forced to reply 3. QxR, QxQ+, 4. Kh2 (forced—and now the priority of check has still kept alive Black’s ability to play…) KxN. Soon Black will be able to play Qxf2 as well. He ends up with a queen, and knight, and a pawn for a rook and a bishop.